Method for Performing a Braking Maneuver Using a Vehicle Braking Assistant

ABSTRACT

The invention relates to a method for performing a braking maneuver using a braking assistant ( 9 ) in a motor vehicle ( 3 ), wherein the braking assistant ( 9 ) initiates an automatic braking maneuver when a distance between the motor vehicle ( 3 ) and an object ( 5 ) located ahead of the motor vehicle ( 3 ) is less than a minimum distance a min , an acceleration signal of the decelerating vehicle ( 3 ) is ascertained, and if the acceleration signal is positive the braking maneuver initiated by the braking assistant ( 9 ) is automatically interrupted.

A driver assistance system is nowadays often part of the standardequipment of a modern motor vehicle. The purpose of such a driverassistance system is to increase highway safety by taking precautions toavoid hazard situations that could lead to accidents. Further aims areto increase comfort by unburdening the driver and to facilitateorientation by providing information on the surroundings that issituation-dependent and makes sense to the driver.

A driver assistance system can intervene semi-autonomously orautonomously in the propulsion, control or signaling devices of thevehicle, warn the driver shortly before or during a critical situationor prevent the latter by independent intervention. Here a driverassistance system is designed so that the responsibility rests with thedriver and the latter can therefore override autonomous interventions.

Here a brake assist system, or “brake assistant” for short, isparticularly important. In hazard situations, in which emergency brakingis necessary, to avoid a collision, the brake pedal is often depressedwith excessive hesitancy. In this case the brake assistant ensuresmaximum brake servo pressure and thus generally the minimum brakingdistance.

Emergency brake assist (EBA), by way of example, detects criticaltraffic situations and ensures optimum braking. To this end the entirearea ahead of the vehicle itself is continuously monitored by means of asuitable sensor for distance and speed measurement, such as for exampleradar, LIDAR (Light Detection And Ranging) or a camera. In this way,other vehicles that are either stationary or moving, or generalobstructions on the highway, are captured. If the distance between thevehicle itself and a vehicle travelling ahead of it falls below a levelthat at the current vehicle speed is deemed critical, the driver isalerted and/or the brake system performs a slight pre-braking, to reducethe braking distance. If the driver then moves their foot from the gaspedal to the brake, the brake assistant simultaneously increases thebraking force. If the driver then actively contributes to the brakingmaneuver, the system provides the necessary braking force to prevent acollision with the vehicle travelling ahead or with other highway users.

If a driver encounters a situation in which, by way of example due toinattentiveness or an unexpected obstruction, they can no longer brakeindependently and if a collision with an unexpected obstruction isimminent, by means of an autonomous full braking function (autonomousemergency braking—AES) a braking assistant can initiate a fullyautomatic emergency braking. In this way, the accident can either becompletely avoided or at least the seriousness of it lessened.

However, such a full braking function becomes particularly problematicalif the direction of travel of the vehicle itself (ego-vehicle) isunknown. If another vehicle ahead of the ego-vehicle is reversing, thento avoid a collision it is also necessary to reverse. But if thebackward-rolling vehicle ahead is rolling faster than the vehicle itselfis travelling or rolling, the distance between the vehicles lessens andin the worst case falls below the specified critical minimum level. Inthis case a braking assistant, having no knowledge of the direction oftravel, commences a braking maneuver, so that the distance between thetwo vehicles undesirably reduces further. In such a situation, acollision is almost unavoidable.

The first object of the invention is to indicate a method allowing abraking assistant, even if it does not know the direction of travel ofthe vehicle, to reduce the danger of, or ideally completely avoid, acollision with other highway users.

The first object is achieved according to the invention by a method forperforming a braking maneuver using a motor vehicle braking assistant ina vehicle, wherein the braking assistant initiates an automated brakingmaneuver when a distance between the motor vehicle and an object locatedahead of the vehicle is less than a minimum distance a_(min), wherein anacceleration signal of the decelerating vehicle is ascertained, andwherein if the acceleration signal is positive the braking maneuverperformed by the braking assistant is automatically interrupted.

In a first step the invention is based initially on the concept that toavoid the problem described above the direction of travel needs to becaptured. This could take place, by way of example, by means ofreversing sensors. But with such reversing sensors, which provideinformation that the reverse gear is selected, there is a danger ofmisinterpretation. Such misinterpretations can, by way of example, occurif a driver allows their vehicle to roll backwards, without selectingreverse gear. The direction of travel will also be falsified if a driverselects reverse gear but allows the vehicle to roll forwards bydepressing the clutch.

Alternatively, what are known as intelligent wheel sensors could be usedwhich, indicate not only the speed of rotation but also the direction ofrotation thereby allowing a clear identification of the direction ofmovement. Due to the high costs, such wheel sensors are only used inhigh-end vehicles.

A surprising finding by the invention in a second step is that it ispossible, even if the direction of travel is not known, to provide areliable and inexpensive solution for avoiding a collision if anacceleration sensor is used. With the acceleration signal, in aparticularly simple way using its sign, the direction of travel of thevehicle can be ascertained and, in the event of an impending collision,a braking maneuver previously initiated automatically by a brakingassistant is interrupted. Thus, a collision due to rolling backwards canbe easily avoided or at least the damage limited.

To this end the acceleration signal of the vehicle is suitably evaluatedtaking account of the sign, wherein essentially a distinction can bemade between two possibilities.

If a vehicle equipped with a braking assistant is travelling in theforwards direction behind another vehicle and if the other vehiclebegins to brake, then the distance between the vehicles reduces and thebraking assistant initiates an automatic braking maneuver when thedistance becomes less than a specified minimum. During a correspondingbraking maneuver from the forward travel, the acceleration signal takesa negative value, meaning that the speed of the vehicle in the forwardsdirection reduces (deceleration). The braking maneuver is accordinglycontinued until the distance between both vehicles again corresponds tothe minimum distance. In this case the braking assistant does notinitiate the direct interruption of the braking maneuver. The brakingmaneuver is deliberate and leads to an intended result, namely areduction in the distance.

If, instead, two vehicles moving backwards are considered, moving atdifferent speeds, in a conventional braking assistant with no knowledgeof the direction of travel there is an increased danger of a collision.If the other vehicle ahead of the ego-vehicle is rolling back morequickly than the ego-vehicle itself, then here also the distance betweenthe two vehicles reduces until after a certain time it is below theminimum distance. Without knowing the direction of travel, the brakingassistant automatically initiates a corresponding braking maneuver,which in the worst case leads to a collision.

Unlike a braking maneuver from forward travel, however, with a brakingmaneuver from backward travel the speed in the backwards directionreduces, and the acceleration signal during a braking maneuver from abackward movement takes a correspondingly positive value. If thispositive value is ascertained as an acceleration signal, thus a gain inspeed is captured, then the braking maneuver is automaticallyinterrupted by the braking assistant. In this case the discontinuationof the braking maneuver only leads to the desired result, namelyreducing the distance.

Overall, the method thus allows, irrespective of direct knowledge of thedirection of travel of a motor vehicle, safe and reliable functioning ofa braking assistant. Knowledge of the direction of travel is obtainedfrom the sign of the acceleration during an initiated braking maneuver.

Essentially the acceleration signal can be obtained with the help of themost varied of sensors or cameras, such as for example with the help ofwhat is known as the egomotion camera from the optical flow or similar.

In an advantageous configuration of the invention, the accelerationsignal is ascertained by means of an onboard acceleration sensor. Suchan acceleration sensor provides reliable results and can also be used inbudget-priced vehicles in the low-cost segment, so that a brakingassistant is no longer used only in high-price vehicles. By way ofexample, acceleration sensors are used to trigger the airbag and wherepiezo technology is used are relatively cheap. The acceleration signalsare accessible via the vehicle CAN (Controller Area Network). Theacceleration signal can be used for evaluating the change in speed.

Advantageously, the current speed change of the vehicle is ascertainedbased on the acceleration signal. It is particularly advantageous if thecurrent change in speed of the vehicle is ascertained by integration ofthe acceleration signal. Integrating the acceleration signal toascertain the change in speed allows an erroneous response to be avoidedto the acceleration peaks that can occur during high-frequency scanningwith both positive and negative signs, even if the entire processoverall only takes place in one direction.

The value of the integrated speed can particularly be used as acriterion for maintaining or interrupting the intervention, so that inthe event of erroneous braking in the backward movement the danger to anoccupant is minimized. To this end, a speed threshold can be specifiedwhich is compared with the integrated speed value.

The acceleration signal can be integrated in various ways. Here theadvantage of integration is the smoothing of the acceleration signal andthus the minimization of an incorrect response. For accelerationintegration, a window integral with low-pass function is preferablyused. Here the area below the window width of the window integralrepresents a speed value which is used to evaluate the situation.

It is also advantageous if a continuous integrator is used as anacceleration integrator. This continuous integrator is continuously“cleared” with a constant speed value and reset to zero, wherein theclearing takes place so that for a positive content of the integrator aconstant value is deducted and for a negative content of the integratora constant value is added. In both types of integration, thus both for apositive and a negative content, the interruption criterion is met if,despite a braking maneuver, in the integral of the acceleration signal again in speed is established which is above a pre-adjustable value.

In the following exemplary embodiments of the invention are explained inmore detail using a drawing. This shows as follows:

FIG. 1 a first traffic situation with two vehicles following one behindthe other and observing a specified minimum distance a_(min), and

FIG. 2 a second traffic situation with the two vehicles and where thedistance is less than the minimum a_(min).

FIG. 1 shows a traffic situation 1, in which an ego-vehicle 3 and anobject located ahead of the ego-vehicle 3, another vehicle 5, aretravelling one behind the other in a lane 7 in the same direction ofmovement x, in forwards direction, at a distance a. The distance betweenthe two vehicles is above a specified minimum distance a_(min)(a>a_(min)).

The ego-vehicle 3 is equipped with a braking assistant 9 with anacceleration sensor 11. By means of the braking assistant 9, in theevent of an impending collision, when therefore the other vehicle 5brakes, a braking maneuver of the ego-vehicle 3 is initiated. Here theacceleration sensor 11 ascertains the acceleration of the ego-vehicle 3.The braking assistant 9 uses the sign of the ascertained accelerationsignal to ascertain the direction of movement of the ego-vehicle 3.

In this case, when the other vehicle 5 brakes, the distance a betweenthe two vehicles 3,5 reduces and the braking assistant 9 initiates abraking maneuver, to restore the required minimum distance a_(min). Thespeed of the ego-vehicle 3 reduces in the forwards direction. Theacceleration signal accordingly takes a negative value. The brakingmaneuver is accordingly continued until the minimum distance a_(min) hasbeen achieved again.

In the traffic situation 21 shown in FIG. 2, in addition to the twovehicles 3, 5 there is an obstruction in the form of a further vehicle23 in the lane 7. The other vehicle 5 located ahead of the ego-vehicle 3is accordingly travelling backwards to avoid the obstruction 23. At thesame time the ego-vehicle 3 is also travelling backwards, to avoid acollision with the other vehicle 5.

In this case, however, the other vehicle is travelling backwards fasterthan the ego-vehicle 3, so that the distance a between the two vehicles3, 5 falls to a level below the critical distance a_(min) (a<a_(min)).Because the level is below the minimum distance a_(min) the brakingassistant 9, which has no knowledge of the direction of travel, thus inthis case the backwards direction, initiates a braking maneuver, toincrease the distance between the two vehicles 3, 5.

The response by the braking assistant 9 would thus lead to the actualcollision between the two vehicles 3, 5 that is to be avoided. Throughfurther intervention of the braking assistant 9, however, such acollision is prevented. By ascertaining the acceleration, the startingdirection of movement of the vehicle 3 is ascertained. The accelerationsignal ascertained by means of the acceleration sensor 11 in this casetakes a positive value during the braking maneuver. In this case theinitiated braking maneuver is interrupted by the braking assistant 9 andthus a collision between the two vehicles 3,5 is avoided or at least theseriousness of the consequences of such a collision is reduced.

LIST OF REFERENCE NUMERALS

1 Traffic situation

3 Ego-vehicle

5 Object

7 Lane

9 Braking assistant

11 Acceleration sensor

21 Traffic situation

23 Obstacle

1. A method for performing a braking maneuver using a braking assistant(9) in a motor vehicle (3), wherein the braking assistant (9) initiatesan automatic braking maneuver when a distance between the motor vehicle(3) and an object (5) located ahead of the motor vehicle (3) is lessthan a minimum distance a_(min), wherein an acceleration signal of thedecelerating vehicle (3) is ascertained, and wherein if the accelerationsignal is positive the braking maneuver performed by the brakingassistant (9) is automatically interrupted.
 2. The method according toclaim 1, wherein the acceleration signal is ascertained by means of anonboard acceleration sensor (11).
 3. The method according to claim 1,wherein the current change in speed of the vehicle (3) is ascertainedbased on the acceleration signal.
 4. The method according to claim 1,wherein the current change in speed of the vehicle (3) is ascertained byintegration of the acceleration signal.
 5. The method according to claim4, wherein, to ascertain the current change in speed of the vehicle (3),an acceleration integrator is used.
 6. The method according to claim 5,wherein, as the acceleration integrator, a low-pass filter is used. 7.The method according to claim 3, wherein the current change in speed ofthe vehicle (3) is compared with a specified speed threshold.
 8. Themethod according to claim 7, wherein, if the speed threshold isexceeded, the braking maneuver of the vehicle (3) is automaticallyinterrupted.
 9. A method of braking a motor vehicle with an automaticbraking assistant system of the vehicle, comprising steps: a) initiatingan automatic braking of the motor vehicle with the automatic brakingassistant system; b) while the automatic braking is proceeding, with asensor of the motor vehicle producing a sensor signal that is indicativeof an acceleration of the motor vehicle, and evaluating the sensorsignal to determine a sign of the acceleration; and c) when the sign ofthe acceleration is positive while the automatic braking is proceeding,then in response thereto automatically interrupting the automaticbraking of the motor vehicle with the automatic braking assistantsystem.
 10. The method according to claim 9, further comprising thestep: d) when the sign of the acceleration is negative while theautomatic braking is proceeding, then in response thereto automaticallycontinuing the automatic braking of the motor vehicle with the automaticbraking assistant system.
 11. The method according to claim 9, whereinthe step a) comprises detecting a spacing distance between the motorvehicle and an object located in front of the motor vehicle, comparingthe detected spacing distance to a distance threshold, and when thedetected spacing distance falls below the distance threshold then inresponse thereto performing the initiating of the automatic braking ofthe motor vehicle.
 12. The method according to claim 9, excludingdetermining a direction of travel of the motor vehicle other than asindicated indirectly by the sign of the acceleration.